This invention relates to molding of thermoplastic materials, and more particularly, to articles useful as molds for such materials and methods for molding employing such articles.
Various types of molds have long been in use for preparing shaped articles from thermoplastic resins, in such operations as blow molding, compression molding, injection molding and injection compression molding. Molds for these purposes are typically manufactured from metal or a similar material having high thermal conductivity.
Blow molding involves the extrusion of a molten tube of resin called a parison into a mold. The mold closes around the parison, pinching the bottom of the parison closed. A gas such as air is then introduced causing the tube to expand against the cool surfaces of the mold. When the parison comes into contact with the cool mold surface, the plastic at the surface quickly freezes. This results in surface imperfections such as die lines, fold lines, pores and voids.
In compression molding, composite blanks of glass reinforced thermoplastic sheets are heated. The material is heated above its melting point or if an amorphous material at least substantially above its glass transition temperature. When the composite blanks are heated, they expand (loft) due to the recoil forces within the fibers. The hot blanks are then pressed between cool mold surfaces which are below the melting point or glass transition temperature. Contact with the cool mold surfaces results in frozen resin on the surface of the blank. This creates unfilled areas in the form of exposed fibers and surface porosity. The resin at the cold surface is frozen and does not flow. Thus, rough boundaries between the charged and newly formed areas are produced.
Injection molding involves injecting molten thermoplastic resin into a mold apparatus. Molds for injection molding of thermoplastic resin are usually made from metal material such as iron, steel, stainless steel, aluminum alloy or brass. Such materials are advantageous in that they have high thermal conductivity and thus allow the melt of thermoplastic resin to cool rapidly and shorten the molding cycle time. Rapid cooling, the injected resin freezes instantaneously at the mold surface, resulting in a thin sold layer which restricts the flow of the molten material.
Rapid quenching and resulting reduced flow of the melt at the mold surface creates several problems, particularly when molding resins which contain large amounts of fillers in the form of fibers and powders. The freezing of these materials at the mold surfaces creates rough surfaces such as exposed fillers, voids and porosity. The quick solidification of the melt combined with limited flowability of the materials makes it difficult to achieve melt flow over a large area. This is especially troublesome when producing thin parts. The use of multiple gates for large and often complex mold cavities produces weld lines where flow fronts meet, which are unsightly and weak. Another important consideration in injection molding of high quality parts is the residual stresses in the molded parts. Residual stress inside a part can result in dimensional instability over the lifetime of the part. Non-uniform residual stresses are often characterized by non-uniform or severe birefringence. Dimensional stability and uniformity of refractive indices are required for high quality parts.
In injection compression molding which is a combined process, a hot thermoplastic melt is injected into a mold cavity. The parting line of the mold is positioned open or allowed to be forced open by the injected melt typically 0.05" to 0.3" inches. The clamping force is increased initiating the compression stroke of the mold forcing the melt to fill the cavity. In many instances the velocity of the melt front through the cavity changes as the injection stroke stops and the compression stroke begins. This distinct change in melt front velocity is often characterized by a stall followed by a surge in the melt front.
The melt begins to quench on the cavity walls as it is injected into the mold. As the melt front stalls, at the completion of injection, and then surges forward, upon the initiation of compression, a blemish, sometimes referred to as a halo, may be produced in the surface of the molded article. The blemish is the result of differential cooling and shear stress which occurs in the injection compression process as a result of the melt front velocity change.
There have recently been disclosed multilayer molds in which a metal core has an insulating layer bonded thereto, for the purpose of slowing the initial cooling of the resin during the molding operation. The insulating layer is fabricated of material having low thermal conductivity, thus slowing the cooling of the molten resin, and also having good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. Said layer may be made of a resin such as polyimide, polyamideimide, polyethersulfone or polyetherketone, typically applied in uncured form (e.g., as a polyamic acid in the case of a polyimide or polyamideimide) and subsequently cured. Cured resins in a solvent carrier may now be employed.
Resinous insulating layers have a major disadvantage, however, in that they are not mechanically strong and are easily abraded upon contact, for example, with filled thermoplastics. Thus, they may not have sufficient mechanical integrity to produce molded articles having surfaces of high quality.
The problem may be remedied, in part, by the application of one or more skin layers of hard material, typically metal, bonded to the insulating layer. The skin layer may be deposited by such operations as electroless deposition, electrolytic deposition and combinations thereof.
Such deposition operations introduce their own problems into the mold fabricating process. It is well known, for example, that the adhesion of metal layers to resinous substrates is poor. This fact has dictated that the resin employed in the insulating layer be one which intrinsically has or can be modified to have relatively high adhesion to metal layers deposited thereon. One genus of resins having this property is the fluorinated polyimides, of the type prepared by the reaction of pyromellitic dianhydride with 2,2-bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane. Such polyimides are available from Ethyl Corporation under the trademark "EYMYD".
Fluorinated polyimides of this type may be subjected to various operations to improve their adhesion to metal. The major disadvantage in the employment of such resins and adhesion improving methods is that the resins are considerably more expensive than corresponding non-fluorinated polyimides and other resins of high thermal conductivity and stability at high temperatures. Moreover, the adhesion improving operation is an additional process step which may be burdensome and inconvenient.
A further problem is the difficulty involved in repairing a mold having a metal skin on a resin insulating layer. To repair even relatively minor damage such as one or more scratches which penetrate the metal skin, it is necessary to remove the mold from use and deposit a new metal layer thereon by further electroless or electrolytic deposition, often after removing at least the area of the old skin which surrounds the damage. Thus, the mold is out of service for a relatively long period, often several weeks.